1. Field of the Invention
The present invention relates to a heat reversible recording medium which is preferable for such an application as rewritable point card and the like, and is capable of rapidly carrying out forming and deleting an image having an excellent visibility. Moreover, the preset invention relates to a heat reversible recording label, a heat reversible recording member, an image processor and an image processing method using the above heat reversible recording medium.
2. Description of the Related Art
A heat reversible recording medium has a heat sensitive layer having a transparency which is reversibly variable depending on temperature. Moreover, the heat reversible recording medium can carry out forming and deleting of an image at an arbitrary timing with ease. With the above, recently the heat reversible recording medium is rapidly becoming more and more prevalent as a rewritable point card and the like. In the present, from the viewpoint of smaller size and lower cost of a recording apparatus, development of the heat reversible recording medium is so desired that the heat reversible recording medium does not need a specific image deleting unit, is capable of forming and deleting the image only with a thermal head and is capable of overwriting.
Conventionally known as the heat reversible recording medium include, for example, the one having organic low molecular compound such as higher fatty acid and the like dispersed in a resin such as vinyl chloride-vinyl acetate copolymer (refer to Japanese Patent Application Laid-Open (JP-A) No. 55-154198). In the above conventional heat reversible recording medium, however, a temperature width showing transparency (light transmission) (as the case may be, hereinafter referred to as “transparency temperature width”) is as narrow as from 2° C. to 4° C., making it uneasy to control temperature when forming the image by using the transparency (light transmission) or whitening property (shading property), which is a problem. For the above problem, a proposal (refer to JP-A No. 2-1363 and JP-A No. 3-2089) is made that uses, as the above organic low molecular compound, a mixture of higher fatty acid and aliphatic dicarboxylic acid, to thereby expand the transparency temperature width to about 20° C., deleting the image (causing transparency). In the above proposal, it is possible to delete the white image (causing transparency) by heating with a heat roller, a heat plate and the like for a comparatively long time. In this case, however, use of the thermal head for heating for a minimum time of several milli seconds may enlarge a temperature distribution in the thickness direction of the heat sensitive layer, thereby a base section which is far from the thermal head cannot be sufficiently heated, failing to sufficiently delete the image, which is a problem.
Another heat reversible recording medium is proposed that is capable of sufficiently deleting the image even when using the thermal head for an overwrite recording. For example, in a heat reversible recording medium (refer to JP-A No. 11-115319), thio ether and aliphatic dibasic acid are contained as the above organic low molecular compound. In this proposal, the transparency temperature width when the heat reversible recording medium is heated for a long time is expanded. In this proposal, however, the minimum heating time of several milli seconds with the thermal head cannot sufficiently delete the image. Moreover, in this proposal, storing the image for a long time at a temperature higher than room temperature with an elapsed time after the image formation may change the deleting energy, making it difficult to delete the image, failing to obtain sufficient deleting property and contrast, which is a problem.
Moreover proposed include a method of containing aliphatic thio ether as an organic low molecular compound (refer to JP-A No. 2000-71623), and a method of containing higher fatty acid ester and aliphatic dibasic acid as an organic low molecular compound (refer to JP-A No. 2000-71624). The above proposals use a resin having sufficiently higher glass transition temperature than a crystallization temperature of the organic low molecular compound. Thereby, heating with the thermal head for a minimum time of several milli seconds cannot sufficiently soften the resin, failing to sufficiently delete the image. Moreover, in the above proposals, storing the image for a long time at a temperature higher than room temperature with an elapsed time after the image formation may change the deleting energy, making it difficult to delete the image, failing to obtain sufficient deleting property and contrast, which is a problem.
On the other hand, a method of containing, as organic low molecular compound, higher fatty acid hydrazide and aliphatic saturated carboxylic acid (refer to JP-A No. 7-101157), and a method of containing, as organic low molecular compound, fatty acid which has fatty acid ester and steroid skeleton (refer to JP-A No. 8-282131). In the above proposals, however, the transparency temperature range is in the high temperature range, thus a sufficient temperature width is not secured. Therefore, use of the thermal head for heating for a minimum time of several milli seconds cannot sufficiently delete the image. Moreover, in the above proposals, storing the image for a long time at a temperature higher than room temperature with an elapsed time after the image formation may change the deleting energy, making it difficult to delete the image, failing to obtain sufficient deleting property and contrast, which is a problem.
Moreover, disposing a temperature slope relieving layer on a surface of a heat sensitive layer is proposed (refer to JP-A No. 2001-30633). In this proposal, however, the heat sensitive layer has a large thickness. Therefore, use of the thermal head for heating for a minimum time of as several milli seconds cannot sufficiently heat the base section of the heat reversible recording medium, specifically, a side not contacting the thermal head, failing to sufficiently carry out forming and deleting of the image. Moreover, in this proposal, storing the image for a long time at a temperature higher than room temperature with an elapsed time after the image formation may change the deleting energy, making it difficult to delete the image, failing to obtain a sufficient deleting property and contrast, which is a problem.
Moreover, a method of mixing a specific cross-linking resin (refer to JP-A No. 8-72416 and JP-A No. 8-127183), and a method of containing a heat sensitive polymer (refer to JP-A No. 10-100547) are proposed. In the above proposed methods, though the image's deleting property can be improved to a certain extent, heating for a minimum time of several milli seconds with a thermal head having fast image-forming rate cannot obtain sufficient deleting property and contrast. Moreover, in the above proposals, storing the image for a long time at a temperature higher than room temperature with an elapsed time after the image formation may change the deleting energy, failing to obtain sufficient deleting property and contrast, which is a problem.
For the purpose of obtaining a heat reversible recording medium free from the above problems, use of resin having a glass transition temperature lower than that of a resin base material is proposed (refer to JP-B No. 3003745). In this case, however, the image's holding property may be insufficient, and storing the image at a temperature higher than room temperature after forming the image may delete the image, failing to obtain a sufficient contrast, which is a problem.
Moreover, decreasing deterioration of the image's deleting property with an elapsed time after the image formation is proposed by using, as a cross-linking agent, a mixture of chain isocyanate compound and cyclic isocyanate compound (refer to JP-A No. 2000-198274). In the above proposal, the image's deleting property with the elapsed time after the image formation is improved by a static deleting method with a hot stamp and the like. Use of the thermal head for heating for a minimum time of several milli seconds cannot improve the image's deleting property, failing to sufficiently delete the image, which is a problem.
Moreover, decreasing the glass transition temperature by blending a low molecular weight polyester resin in a resin base material at a coagulation point 30° C. or less is proposed (refer to JP-A No. 2000-52662 and JP-A No.2002-113956). In the above proposal, however, the low molecular weight polyester resin may move after forming of the image and thereby the image may be deleted, failing to obtain sufficient contrast, moreover, the low molecular weight polyester resin may be deposited, which are problems.
In sum, such a heat reversible recording medium and such a related art using the above heat reversible recording medium are yet to be provided as can sufficiently delete the image even when the image is heated with the thermal head for a minimum time of several milli seconds, can keep, with the elapsed time after the image formation, a sufficient deleting property and a sufficient contrast without changing the deleting energy, and can form the image excellent in storing property, visibility and the like.